As a rule up to now it was only possible to make repairs to a tunnel drilling machine, in particular in inaccessible tunnel cross sections, if the tunnel drilling machine had been made accessible by additional digging. The removal of the tunnel drilling machine from a tunnel tube is not easily possible, in particular in those cases where safety measures against cave-ins, such as tubbings or cladding with steel rings are required, since it is not always possible to retract the drilling machine through such restrictions of the cross section. Constrictions of the tunnel cross section by convergence can occur with increased rock pressure, which also makes the retraction of the tunnel drilling machine more difficult.
In connection with soft rock and with narrower, inaccessible tunnel cross sections it is known to fold partial areas of the drill head by means of mechanical devices or to reduce its cross section. In general this refers to partial width drilling tools, and different adjustment mechanisms can be found in, for example, U.S. Pat. No. 5,104,262, German Published, Non-Examined Patent Application DE-OS 31 40 203 and German Patent DE-PS 3 219 362. A cutting head which is pivotable to all sides can be found in European Patent Publication EP-B1 169 393, and is movable in a frame in the linear direction of the tunnel tube, this embodiment also being related to a partial width machine. All known devices wherein the tools can be brought into an appropriate position pivotably or together with a cutting head have the disadvantage, particularly in connection with cutting of hard rock, that the additional mechanical devices are subjected to heavy loads and therefore cause an additional susceptibility for breakdown. Incidentally, when cutting hard rock in particular, the employment of full-width drill heads is considerably more advantageous, and up to now the only option for salvaging such full-width drill heads was to dig them out from the surface of the section to be drilled out.